Systems and methods for closed loop control to ensure a constant current output with a changing load resistance

ABSTRACT

A closed loop control system automatically ensures that an output of a device is constant. The system can receive an input to set a fixed value for a variable (e.g., a current, a heart rate, a tissue perfusion, an ion level, etc.), and this variable can be delivered to a feedback component. The system can also include the device to deliver the variable to a load. The feedback component can be coupled to the delivery device to sample the output of the delivery device at different times. Based on the sampling, the feedback component can vary a property of the delivery device related to the delivery of the variable to the load to ensure that the variable remains constant at the fixed value. In some instances, the system can be implemented as a stimulator that delivers the constant current of a current source and has a low output impedance of a voltage source.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/329,343, entitled “SYSTEMS AND METHODS FOR CLOSED LOOP CONTROL,”filed Apr. 29, 2016. This application also claims the benefit of U.S.Provisional Application No. 62/256,758, entitled “CONSTANT CURRENT POWERVOLTAGE SOURCE STIMULATOR”, filed Nov. 18, 2015. The entirety of theseprovisional applications is hereby incorporated by reference for allpurposes.

TECHNICAL FIELD

The present disclosure relates generally to closed loop control, and,more specifically, to systems and methods for automatic closed loopcontrol to ensure that an output of a device (e.g., a current) isconstant when a resistance of a load changes.

BACKGROUND

Electrical stimulation is generally the application of an electricalcurrent to a load using a current source or a voltage source. A voltagesource has a low output impedance and provides a constant voltage andvariable current, while a current source has a high output impedance andprovides a constant current and variable voltage. In applications wherethe load is neural tissue, traditionally, a constant current source hasbeen used to deliver a known constant current in the presence of aconstantly changing tissue resistance. However, application of theconstant current can lead to an accumulation of charge at thetissue-electrode interface, which can cause irreversible electrochemicalreactions that can damage the tissue or the electrode.

SUMMARY

The present disclosure relates generally to closed loop control, and,more specifically, to systems and methods for automatic closed loopcontrol to ensure that an output of a device is constant. In someinstances, the automatic closed loop control can be used with astimulator to control a current being applied to a load (e.g.,biological tissue). An operator (e.g., a clinician) can preset thecurrent, and the closed loop control can automatically control the powerinto the load to ensure that the preset current is delivered to theload. Accordingly, the stimulator can provide a constant current, whileoffering a low output impedance. In other instances, the automaticclosed loop control can be used to ensure that any variable that is setat a fixed level by an input, including heart rate, tissue perfusion,ion level, and the like, remains at the fixed level. As an example, adevice can automatically control heart rate by stimulating the vagusnerve with the heart rate being the feedback variable.

In one aspect, the present disclosure can include a system that ensuresthat an output of a device is constant. The system can include a deviceto deliver a variable to a load. The system can also include a feedbackcomponent, coupled to the delivery device, to sample the output of thedelivery device at different times. Based on the sampling, the feedbackcomponent can vary a property of the delivery device related to thedelivery of the variable to the load to ensure that the variable remainsconstant at the fixed value. The feedback component can receive an inputto set the fixed value for the variable (e.g., a current, a heart rate,a tissue perfusion, an ion level, etc.).

In another aspect, the present disclosure can include a method forensuring that an output of a device is constant. The method includes:setting a constant output to provide to a load, wherein the output isprovided with a low output impedance; sampling the variable associatedwith the output; and controlling a property related to the output basedon the sampled variable so that the output to the load remains constant.In some instances, the method can be used in connection with astimulator device that provides a constant current with a low outputimpedance, achieving properties of both a current source and a voltagesource. For example, the load can include one or more nerves and theoutput can ensure that a constant current is applied to stimulate theone or more nerves with a low output impedance to eliminate theformation of harmful electrochemical reaction products.

In a further aspect, the present disclosure can include a stimulatorwith a constant output current and a low output impedance. Thestimulator can include a voltage source to provide a set point voltage.The stimulator can also include an adjustable gain amplifier to receivethe set point voltage and provide the output current independent of aimpedance of a load. The stimulator can also include a feedbackcomponent, coupled to the adjustable gain amplifier, to sample a powerat different times and vary a gain of the adjustable gain amplifierbased on the sampled power.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present disclosure will becomeapparent to those skilled in the art to which the present disclosurerelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram showing an example of a closed loop controlsystem according to an aspect of the present disclosure;

FIG. 2 is a block diagram showing a stimulator system that can employthe closed loop control system in FIG. 1 to provide a constant currentof a current source and a low output impedance of a voltage source;

FIG. 3 is a block diagram of an example of a feedback circuit that canbe used in the stimulator system in FIG. 2;

FIG. 4 is a block diagram showing a digital implementation of thestimulator system in FIG. 2;

FIG. 5 is a block diagram showing the stimulator system in FIG. 2implemented for magnetic stimulation;

FIG. 6 is a process flow diagram of a method for ensuring that an outputof a device is constant, according to another aspect of the presentdisclosure; and

FIG. 7 is a process flow diagram showing an example method for applyinga constant current for neural stimulation with a low output impedance toprevent the formation of harmful irreversible electrochemical reactionproducts.

DETAILED DESCRIPTION I. Definitions

Unless otherwise defined, all technical terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich the present disclosure pertains.

In the context of the present disclosure, the singular forms “a,” “an”and “the” can also include the plural forms, unless the context clearlyindicates otherwise.

The terms “comprises” and/or “comprising,” as used herein, can specifythe presence of stated features, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, steps, operations, elements, components, and/or groups.

As used herein, the term “and/or” can include any and all combinationsof one or more of the associated listed items.

Additionally, although the terms “first,” “second,” etc. may be usedherein to describe various elements, these elements should not belimited by these terms. These terms are only used to distinguish oneelement from another. Thus, a “first” element discussed below could alsobe termed a “second” element without departing from the teachings of thepresent disclosure. The sequence of operations (or acts/steps) is notlimited to the order presented in the claims or figures unlessspecifically indicated otherwise.

As used herein, the term “closed loop control” refers to automaticchanges to a variable that are made to a variable based on a detectedoutput to ensure that an input remains constant.

As used herein, the term “automatic” refers to a device or processoperating by itself with little or no direct human control.

As used herein, the term “variable” refers to an element that varies orchanges. Examples of variables include, but are not limited to, current,current power, heart rate, tissue perfusion rate, and ion or otherbiometric level.

As used herein, the term “stimulator” refers to an analog or digitaldevice that delivers electrical current to a load. The stimulator canhave characteristics of a current source (constant current application)and a voltage source (low output impedance).

As used herein, the term “load” refers to an element that is connectedto the output of the stimulator. In some instances, the load can includean interface between an electrode and biological tissue (the“electrode-tissue interface”). For example, the biological tissue caninclude one or more nerves.

As used herein, the term “nerve” can refer to one or more fibers thatemploy electrical and chemical signals to transmit motor, sensory,and/or autonomic information from one body part to another. A nerve canrefer to either a component of the central nervous system or theperipheral nervous system.

As used herein, the term “impedance” refers to the measure of theopposition that a circuit presents to a current when a voltage isapplied. The term “resistance” may be used interchangeably withimpedance herein.

As used herein, the term “output impedance” refers to the oppositionexhibited by output terminals of the stimulator device and can be theimpedance looking back into the output terminals.

As used herein, the term “load resistance” refers to the “inputimpedance” or the opposition to current flow from the stimulator device,which can be variable over time. As an example, the load resistance canbe provided by the biological tissue (“tissue resistance”).

As used herein, the term “sampling” refers to the reduction of acontinuous stimulation signal to a discrete value at a point in time.

As used herein, the term “coupled” can refer to two circuit elementsbeing electrically connected to each other so that current flowstherebetween in at least one direction.

As used herein, the terms “subject” and “patient” can be usedinterchangeably and refer to any warm-blooded organism including, butnot limited to, a human being, a pig, a rat, a mouse, a dog, a cat, agoat, a sheep, a horse, a monkey, an ape, a rabbit, a cow, etc.

II. Overview

Traditionally, neural stimulation has been conducted with a currentsource to deliver a known current to a continually changing tissueresistance. Generally, a regulated current source includes a voltagesource, a current monitoring circuit, and a feedback circuit. As thetissue resistance changes, the feedback circuit instantaneously adjuststhe voltage to maintain a constant current, thereby generating a highoutput impedance. However, if the adjustment is delayed in time, theoutput impedance can remain low, while maintaining the constant currentdelivered. Advantageously, the present disclosure describes a stimulatorwith characteristics of a current source (constant current application)and a voltage source (low output impedance).

The systems and methods described herein provide automatic closed loopcontrol to ensure that an output of a device (e.g., the current providedby a stimulator) is constant. The stimulator also provides a low outputimpedance to reduce the potential for damage at the electrode-tissueinterface due to irreversible electrochemical reaction products. Asopposed to the regulated current source, the feedback circuit of thestimulator described herein does not take instantaneous currentmeasurements. Instead, the current measurements are taken over a periodof time to estimate the power of the current delivered, which accountsfor a change in resistance of the load. The stimulator has applicationwhere the load resistance changes slowly and the circuit does notrequire instantaneous adjustment. The stimulator can operate as ananalog or digital electrical circuit or even find application as amagnetic stimulator. Moreover, the automatic closed loop control of thesystems and methods described herein can be used to control outputs andvariables other than current, such as heart rate, tissue perfusion rate,ion or other biometric level, or the like.

III. Systems

One aspect of the present disclosure can include a closed loop controlsystem 10 (FIG. 1) that can ensure that an output of a delivery device12 is constant. The system 10 can include an input that can set a fixedvalue for a variable (e.g., a current, a heart rate, a tissue perfusion,an ion level, etc.). As an example, the input can be set by a user(e.g., a clinician) to a certain value. In another example, the inputcan be set by the system 10 based on one or more characteristics of aload. The system 10 can also include a delivery device 12 to deliver asignal based on the variable as an output. In some examples, the outputcan be delivered to the load. The system 10 can also include a feedbackcomponent 14, coupled to the delivery device 12, to sample the output ofthe delivery device 12 at different times. In some instances, thesampling is not done instantaneously and, instead, is done over a windowof time. Based on the sampling, the feedback component 14 can vary aproperty of the delivery device 12 related to the delivery of the signalto the load to ensure that the variable remains constant at the fixedvalue. In some instances, the system 10 can ensure that the output doesnot vary when a resistance of the load changes.

In some instances, system 10 can be used with a stimulator to control acurrent being applied to a load (e.g., biological tissue). An operator(or clinician) can preset the current and the system 10 canautomatically control the power into the load to ensure that the presetcurrent is delivered to the load. Accordingly, the stimulator canprovide a constant current, while offering a low output impedance. Inother instances, the system 10 can be used to ensure that any variablethat is set at a fixed level by an input, including heart rate, tissueperfusion rate, ion or other biometric level, and the like, remains atthe fixed level. As an example, the delivery device 12 can automaticallycontrol heart rate by stimulating the vagus nerve, and the variable fedinto the feedback component 14 can be a resulting heart rate.

As shown in FIG. 2, the closed loop control can be used with astimulator 20 that can provide an analog current (Io) to a load. Thestimulator 20 can provide a constant current of a current source and alow output impedance of a voltage source. The stimulator 20 can includea voltage source 22, a variable gain amplifier 24, and a feedbackcircuit 26. The feedback circuit 26 can measure the current through theload over a window of time to estimate the power of the current over thewindow of time. The feedback circuit 26 does not measure aninstantaneous amplitude of the current, like a traditional currentsource with infinite output impedance. Instead, the power estimate overthe time window enables the output impedance of the stimulator 20 toremain low, but ensures the delivery of the constant current.

Advantageously, the stimulator 20 can be used in neural stimulationapplications, especially using high frequency alternating current (HFAC)waveforms. The stimulator 20 can deliver a known, constant current tothe tissue in the presence of a changing tissue resistance. Thestimulator 20 can also reduce the accumulation of charge at thetissue-electrode interface, which is common in traditional neuralstimulation applications. For example, the reduction of chargeaccumulated at the tissue-electrode interface can reduce the occurrenceof irreversible electrochemical reactions that can damage the electrodeor the tissue.

As shown in FIG. 2, the voltage source 22 feeds a set point voltageinput (Vo) into a variable gain amplifier 24. The variable gainamplifier 24 is connected to the load to supply a known input current(Io) to the load. For example, the input current (Io) can be selected bya user (e.g., a clinician) for a certain neural stimulation application.The load can be associated with a load resistance (Ro) that may change.The feedback circuit 16 can determine a change in the load resistance(Ro) and control the gain (G) of the variable gain amplifier 24accordingly to ensure that the input current (Io) supplied to the loaddoes not vary. In these instances, the load resistance (Ro) is assumedto change more slowly than a rate of integration of the feedback circuit26.

An example of the feedback circuit 26 is shown in FIG. 3. The feedbackcircuit 26 can include a resistor in series with the load, or seriesresistance (Rs), to sample the current through the load. The currentthrough Rs is then amplified with a differential amplifier with a gainB. The output of the amplifier B is rectified and integrated to obtain avoltage V_(RMS) related to the power (RMS) of the current into the load.For example:V _(RMS) =B I _(RMS) Rs,where I_(RMS) is the current related to the power (RMS) of the currentinto the load.

The voltage V_(RMS) can be compared to a preset control value (Vc). Forexample, the present control voltage value (Vc) can be the same as theconstant set point voltage (Vo) supplied by the voltage source 22.However, the preset control value (Vc) need not be the same as theconstant set point voltage (Vo). The difference between the voltageV_(RMS) and the present control voltage value (Vc) is amplified by anamplifier with a very large gain A (much larger than G/KBRs) to generatea voltage V_(G), which is used to control the gain (G) of the variablegain amplifier 24. For example:G=KV _(G),where K is a constant.

The current related to the power (RMS) of the current into the load (oroutput current power) is independent of the load resistance, as:I _(RMS) =Vc/B Rs,so that I_(RMS) is constant and proportional to Vc when A is very large,such that G/KBRs is much less than A. B and Rs are fixed at constantvalues. Indeed, I_(RMS) is also independent of the load resistance.

As shown in FIG. 4, the stimulator 40 can be implemented digitally. Thedigital implementation of the stimulator 40 can include a microprocessor42, a digital-to-analog converter (D/A) 44, and an analog-to-digitalconverter (A/D) 46. The microprocessor 42 can set a voltage (e.g., Vc)required at the input of the D/A 44. The voltage output is applied tothe load. The load can have a resistance Ro. The current through theload is measured by sampling the voltage through the load. For example,as illustrated in FIG. 4, the sampling can be done by the seriesresistor (Rs) and one or more amplifiers with various gains. In someinstances, the amplifiers can be the same as those shown in FIG. 3. Thepower of the signal is calculated by the microprocessor and the power iscompared to the required voltage. The microprocessor 42 can change aproperty related to the required voltage based on the comparison.

The stimulator 50, as shown in FIG. 5, can be implemented for magneticstimulation where it is difficult to maintain the generated inducedpower. In particular, any change in the angle of the coil (e.g.,stimulator coil) or the distance to the target load can be changing. Themagnetic stimulator 50 can include a variable voltage source 52 (whichcan be large) and a switch 54 to discharge the charge from the voltagesource 52 to a coil for stimulating (stimulator coil). A monitoring coilor other sensor can detect the induced electric field at or close to thedesired target and in the correct direction. For example, the monitoringcoil can be a single turn coil. In other examples, other sensors can beused, such as a Hall sensor or a GMR sensor. The output of the sensorcan be amplified, and the RMS value of the signal can be integrated toobtain the power of the induced electric field. The RMS value can becompared to the reference value (Vc) and the voltage source 52 can beadjusted accordingly. The magnetic stimulator 50 can be controlled by adifferent variable than the induced magnetic power. For example, themagnetic stimulator 50 can be applied to lower neural excitability ofthe brain and using the feedback can be a preset level of neuralexcitability.

IV. Methods

Another aspect of the present disclosure can include a method 60 (FIG.6) for ensuring that an output of a device (e.g., a current, a heartrate, a tissue perfusion, an ion level, or the like) is constant. Themethod 60 can provide for closed loop control, by which a variableassociated with the output is detected (e.g., by feedback component 14)and a characteristic associated with the input is adjusted (e.g., withindelivery device 12) based on the detected variable associated with theoutput. Advantageously, the method 60 can be used to provide astimulator device that can provide a constant current like a currentsource, while providing a low output impedance like a voltage source.

The method 60 can generally include the steps of: setting a constantoutput to provide to a load (Step 62); sampling a variable associatedwith the output at different times (Step 64); and controlling a propertyrelated to the output based on the sampled variable so that the outputto the load remains constant (Step 66). The method 60 is illustrated asprocess flow diagrams with flowchart illustrations. For purposes ofsimplicity, the method 60 is shown and described as being executedserially; however, it is to be understood and appreciated that thepresent disclosure is not limited by the illustrated order as some stepscould occur in different orders and/or concurrently with other stepsshown and described herein. Moreover, not all illustrated aspects may berequired to implement the method 60.

At Step 62, a constant output is set to be provided to a load. Theoutput can be set to a fixed value for a variable, such as current,heart rate, tissue perfusion, ion level, or the like. As an example, theinput can be set be a user, such as a clinician, to a certain fixedvalue. In another example, the input can be set automatically based onone or more characteristics of a load. The output can be provided to theload by a device (e.g., delivery device 12 or any type of stimulator20-50).

At Step 64, a variable associated with the output is sampled atdifferent times (e.g., by a feedback component 14 coupled to thedelivery device 12). In some instances, the sampling is not doneinstantaneously and, instead, is done over a window of time. At Step 66,a property related to the output can be controlled (e.g., by a portionof the delivery device 12 based on a determination by the feedbackcomponent 14) based on the sampled variable so that the output to theload remains constant at the fixed value. In some instances, when aresistance of the load changes the output does not vary.

An example method 70 for applying a constant current to a nerve with lowoutput resistance is shown in FIG. 7. The method 70 can be executed, forexample, by the stimulator shown in FIG. 2. At Step 72, a constantcurrent to be provided to biological tissue including one or more nervescan be set. In some instances, the constant current can be set inresponse to an input from a user (e.g., a clinician). In otherinstances, the constant current can be set automatically. The currentcan be provided by the stimulator including a voltage source 22 and anassociated variable gain amplifier 24, as shown in FIG. 2.

At Step 74, the one or more nerves can be stimulated with the constantcurrent. Advantageously, with the arrangement shown in FIG. 2, theconstant current can be delivered without the generation of harmfulreaction products due at least in part to a low output impedance of thevoltage source 22 of the stimulator. At Step 74, feedback can beprovided related to a change in tissue resistance due to thestimulation. For example, the feedback can be provided by a feedbackcircuit 26 shown in FIG. 3. The feedback can include a sampling of apower delivered to the biological tissue, and comparing a RMS voltage toa constant set point voltage. At 78, a parameter (e.g., a gain of thevariable gain amplifier 24) related to the current can be changed inresponse to the feedback. Changing the parameter can ensure that theconstant current is applied to the biological tissue, even when thetissue resistance changes. The sampling is conducted over a time window,instead of instantaneously, assuming that the tissue resistance changesmore slowly than an integration time of the feedback.

From the above description, those skilled in the art will perceiveimprovements, changes and modifications. Such improvements, changes andmodifications are within the skill of one in the art and are intended tobe covered by the appended claims.

The following is claimed:
 1. A stimulator with a low output impedance,the stimulator comprising: a voltage source configured to generate aninput voltage waveform; a variable gain voltage amplifier configured toreceive the input voltage waveform from the voltage source and providean output voltage to a load; and a feedback component, coupled to acurrent return of the adjustable gain amplifier, comprising a set pointvoltage source configured to generate a set point voltage and anamplifier configured to compare an output of a current power detectorcircuit to the set point voltage, wherein the feedback component isconfigured to determine the voltage delivered to the load by samplingcurrent into the load over a time period, estimating a power based onthe current into the load over the time period, and wherein a gain ofthe adjustable gain amplifier is adjusted based on a difference betweenthe output of the current power detector and the set point voltage sothe current power output to the load remains constant.
 2. The stimulatorof claim 1, wherein the power is estimated by calculating a root meansquare of the current into the load.
 3. The stimulator of claim 1,wherein the current power detector comprises a resistor in series withthe load to sample the current into the load and a differentialamplifier with a constant gain to determine the output of the currentpower detector.
 4. The stimulator of claim 1, wherein the circuitfurther comprises a second amplifier configured to receive the outputand the set point voltage and provide the difference between the outputof the current power detector and the set point voltage to adjust thegain of the adjustable gain amplifier.
 5. The stimulator of claim 1,wherein the stimulator is configured to reduce charge accumulation at atissue-electrode interface, wherein the load is tissue.